OPTICALLY STIMULATED LUMINESCENCE DATING OF POSTGLACIAL FLOODPLAIN SEDIMENTS IN IRELAND: IT IS LIKE ASSEMBLING A JIGSAW PUZZLE!

Alluvial deposits are archives of information and are valuable repositories of past environmental change. This study characterizes the evolution of a fluvial sedimentary archive in a temperate humid floodplain on the River Nore (Ireland) using morphological, sedimentological, and geochemical signatures contained therein. Previous attempts at establishing robust chronologies with radiocarbon dating have been hampered by issues related to contamination by old carbon or lack of suitable dating materials. As an alternative, optically stimulated luminescence (OSL) dating came to the rescue!

Seven samples for luminescence dating were retrieved from recently excavated trenches. OSL dating was performed on quartz mineral (150 – 250 µm), where a small amount of quartz was dispensed on each aliquot, and measured with a single aliquot regenerative protocol (SAR). The luminescence response was very dim, as is typically expected from sediment in recently glaciated landscapes. Fortunately, they behaved well.

The sedimentary units are rather thin and judicious sampling was performed in order to properly assess the dose rate. Also, manganese oxide concretions are found in the sections and are found in greater abundance above clay-rich layers. These concretions present a highly elevated activity in radium 226, compared to its parent, uranium 238. The bulk sediment compositions show no such discrepancy. It implies that the oxide concretions formed post sedimentation and are actively absorbing radium. This presents an interesting puzzle, which will be addressed here, in our effort to devise a chronology with luminescence dating.